This invention relates to camera tubes and in particular to camera tubes of the photoconductive target type, such as, for example, vidicon camera tubes.
A typical camera tube of the above type as at present known is illustrated in FIG. 1 which shows a longitudinal cross section of the tube. Referring to FIG. 1 the tube consists of a glass envelope 1 having a face plate 2. On the inner surface of the face plate 2 is the target electrode 3. The customary target ring is referenced 4. At the other end of the tube the base of which is shown broken away is an electron gun 5 which in operation projects an electron beam down the length of the tube towards the target 3.
At the output end of the electron gun 5 are the first and second grids (normally referred to as G1 and G2) here referenced 6 and 7 respectively. Grid 7, an accelerating grid electrode, has therein a beam width defining aperture 8.
Following grid 7 is a cylindrical anode 9 (commonly referred to as G3). At the end of the cylindrical anode 9 towards the target 3 is a mesh 10 normally referred to as the terminating mesh extending across a carrier 11.
While not shown in FIG. 1 the tube is provided to be operated within a solenoid producing a magnetic focusing field in the direction of the axis of the tube.
With a typical vidicon camera tube as illustrated in FIG. 1 the resolution achieved is a function of the electron beam spot size at the target 3. This in turn is related to the size of the defining aperture 8 in the second grid 7 and the demagnification of the image of this aperture at the target 3. Demagnification is a function of H.sub.A /H.sub.T where H.sub.A is the magnitude of the focusing magnetic field at the plane of the aperture 8 and H.sub.T is the magnitude of the focusing magnetic field in the plane of the target 3.
In a uniform magnetic field as normally produced by the focusing solenoid H.sub.A /H.sub.T =1 and therefore no demagnification occurs.